Phase 0: remove auto-updater, pin Go 1.27.1, fix vet warnings, tidy ignores

- Delete BinaryUpdate, internal/up2date, GitHub/Update structs and the
  xteveAutoUpdate / update.url settings (UI rows, en.json, defaults).
  Settings-schema migrations kept and moved to src/migrate.go.
- Drop kardianos/osext dependency.
- xteve.go version 0200 -> 0201 to match changelog; Drone now fails on drift.
- go.mod go 1.27.1; Dockerfile and Drone golang images pinned to 1.27.1.
- Fix four go vet unreachable-code warnings.
- .gitignore: .gocache/, agent.md, skill.md. .dockerignore: build context
  no longer includes caches, ts/, tasks/ or markdown except the changelog.
- Drone: publish :latest only from master; other branches publish a
  branch-named tag so a feature push cannot replace the deployed image.
- Add tasks/improvement-plan.md and tasks/todo.md.
- Regenerate src/webUI.go.

src/info.go

@@ -22,7 +22,6 @@ func ShowSystemInfo() {
 
 	fmt.Println(fmt.Sprintf("Version:             %s %s.%s", System.Name, System.Version, System.Build))
 	fmt.Println(fmt.Sprintf("Branch:              %s", System.Branch))
-	fmt.Println(fmt.Sprintf("GitHub:              %s/%s | Git update = %t", System.GitHub.User, System.GitHub.Repo, System.GitHub.Update))
 	fmt.Println(fmt.Sprintf("Folder (config):     %s", System.Folder.Config))
 
 	fmt.Println(fmt.Sprintf("Streams:             %d / %d", len(Data.Streams.Active), len(Data.Streams.All)))
@@ -68,7 +67,6 @@ func ShowSystemInfo() {
 	println("---")
 
 	fmt.Println("Settings [General]")
-	fmt.Println(fmt.Sprintf("xTeVe Update:        %t", Settings.XteveAutoUpdate))
 	fmt.Println(fmt.Sprintf("UUID:                %s", Settings.UUID))
 	fmt.Println(fmt.Sprintf("Tuner (Plex / Emby): %d", Settings.Tuner))
 	fmt.Println(fmt.Sprintf("EPG Source:          %s", Settings.EpgSource))